FIG. 1 illustrates schematically a conventional way of producing an X-ray image in electronic form. An X-ray tube 101 emits X-rays that propagate through an object under study (such as a part of a human body) 102 and enter an image intensifier 103. Optical equipment 104 convey the output signal of the image intensifier 103 onto a CCD array 105, from which the two-dimensional image is read and stored into a memory 106 and/or shown on a display 107. The drawbacks of the conventional approach arise from the large number of different devices and components that take part in converting the spatial distribution of differently attenuated x-rays into an actual image. Taking the signal through a variety of devices and components reduces the efficiency of collecting information from the object under study and introduces noise.
FIG. 2 illustrates a more recently adopted approach where detected X-rays are converted into an image through fewer intermediate phases. The whole arrangement of image intensifier, optics and CCD is now replaced with a single digital detector 201 the output signal of which is ready to be taken into a memory 106 and/or a display 107. The structure and operation of the digital detector 201 are thus such that the spatial distribution of X-ray intensity that hits the detector 201 can be more or less directly converted into an array of digital pixel values. Truly digital imaging of this kind can make the process of producing X-ray images in electronic form remarkably simpler and more efficient compared to the conventional approach of FIG. 1.
There remains the question of how should the digital detector 201 be constructed. At the priority date of this description the most well-known example of digital X-ray imaging detectors is the CsI/Si (Cesium Iodide/Silicon) detector of GE Medical Systems Corporation. FIG. 3 is a simplified cross-section of a CsI/Si detector panel 301. A CsI scintillator layer 302 is arranged to meet the incoming X-ray photons. Absorption of a photon into the CsI layer 302 causes light to be emitted locally into an amorphous Si panel 303 that is located immediately next to the CsI layer 302 and constitutes a two-dimensional photodiode/transistor array. It absorbs the light and converts it into electronic charge. Each photodiode in the amorphous Si panel 303 represents a pixel in the image to be generated. An arrangement of low-noise readout electronics 304 behind the amorphous Si panel 303 is employed to collect the accumulated charge from each photodiode and convert the individual quantities of collected charge into corresponding digital value. The two-dimensional constellation of digital values represents an image that can be stored into memory and/or shown on a display.
A critical factor of a detector of the type shown in FIG. 3 is the scattering of light between the CsI scintillator layer 302 and the amorphous Si panel 303. The objective of producing sharp images calls for keeping each burst of scintillation photons confined into an as small spatial sector as possible. GE Medical Systems Corporation announces having developed a “needle-like” structure for the CsI layer that should prevent scattering to a large extent. However, producing such a structure with good yield and highly homogenous gain throughout the detecting surface may be problematic. Homogeneity of the detector response is of crucial importance for example in medical imaging applications where far-reaching decisions are made on the basis of what fundamentally is a detected spatial distribution of received X-ray intensity. Another disadvantage of the structure of FIG. 3 are that only approximately one half of the visible light photons can be collected. Amorphous Si is not known to be a very good photodetector; it is used in this structure mainly because it allows building a detector with a relatively large area. Siemens Corporation has a corresponding product on the market with the trade name TRIXELL.
Other known techniques for obtaining digital X-ray images include slot scanning, where a linear detector is mechanically moved across the illumination beam; using tiled CCD arrays coupled to a scintillator plate via fiber optics; computed radiography where electrons are trapped on photostimulated plates that are then exposed to generate image data; and direct conversion. The last-mentioned has traditionally meant that two-dimensional Selenium detector panels are used for receiving the X-ray photons, which get absorbed and give rise to local accumulation of charges in the bulk of the Se substrate. Readout electronics are then employed to collect the accumulated charge and to convert the collected charge values into a two-dimensional image. The drawbacks of the Se-based direct conversion detector arrangements have been associated with questionable reliability as well as a relatively low DQE (Detective Quantum Efficiency) values, which cause degradation to image quality and preclude the use of Se-based direct conversion detectors in advanced applications of X-ray diagnostics and therapy.